1. Field of the Invention
The present invention generally relates to a controller for an electric power steering system adapted to give a steering system of an automobile or a vehicle a steering assist force which is generated by a motor. More specifically, the present invention relates to a controller for an electric power steering system adapted to enhance the responsiveness thereof to an emergency steerage (or steering operation) by predicting an occurrence of a drop in power supply voltage and by performing an idling-up operation.
2. Description of the Related Art
Conventional motor-driven power steering system for performing the auxiliary load pressing of a steering unit by utilizing the turning (or rotating) force of a motor is adapted so that the auxiliary load pressing is performed on a steering shaft or a rack shaft by means of a transmitting mechanism, such as a gear or a belt, for transmitting the driving force of the motor through a reduction gear. Such a conventional motor-driven power steering system performs feedback control of the motor current so as to generate steering assist torque accurately.
The feedback control is to regulate a voltage applied or impressed to the motor in such a manner that the difference between a current control value and a motor current detection value decreases. Generally, the regulation of the voltage applied to the motor is performed by regulating a duty ratio to be used in PWM (Pulse Width Modulation) control operation.
Hereinafter, the common or ordinary configuration of such a motor-driven power steering system will be described by illustrating thereof in FIG. 1. A shaft 2 of a steering handle 1 is connected with tie-rods 6 respectively corresponding to wheels through a reduction gear 3, universal joints 4a and 4b and a pinion/rack mechanism 5. A torque sensor 10 for detecting the steering torque of the steering handle 1 is attached to the shaft 2. A motor 20 for assisting the steering force of the steering handle 1 is connected to the shaft 2 through a clutch 21 and the reduction gear 3. Electric power is supplied through a battery 14 and an ignition key 11 to a control unit 30 for controlling the power steering system. Then, the control unit 30 computes a steering assist command value I, which is obtained by performing an assist command, on the basis of steering torque T, which is detected by the torque sensor 10, and of a vehicle velocity or speed V, which is detected by a vehicle speed sensor 12. Further, on the basis of the computed steering assist command value I, the control unit 30 controls an electric current to be supplied to the motor 20. On/off control of the clutch 21 is controlled by the control unit 30 and is turned ON (namely, is engaged) under ordinary operating conditions. Moreover, when the control unit 30 judges that the power steering system is out of order, and when power from the battery 14 is turned OFF by the ignition key 11, the clutch 21 is turned OFF (namely, is disengaged).
The control unit 30 is constituted mainly by a CPU. FIG.2 illustrates the common functions to be performed by executing programs in the CPU. For example, a block, in which a word "phase compensator 31" is written, does not represent a phase compensator serving as independent hardware. Instead, this block represents a phase compensation function to be performed by the CPU. Functions and operations of the control unit 30 will be described hereinbelow. First, in the phase compensator 31, a phase compensation is performed on the steering torque T, which is detected and inputted by the torque sensor 10, so as to enhance the stability of the steering system. Subsequently, the phase-compensated steering torque TA is inputted to a steering assist command value computing unit 32. Further, a vehicle speed V detected by the vehicle speed sensor 12 is inputted to the steering assist command value computing unit 32. Then, the steering assist command value computing unit 32 determines a steering assist command value I, which is a control target value of a current to be supplied to the motor 20, according to the steering torque TA and the vehicle speed V inputted thereto. Additionally, a memory 33 is attached to the steering assist command value computing unit 32. Further, the memory 33 stores the steering assist command value I corresponding to the steering torque therein by using the vehicle speed V as a parameter, and is used for computing the steering assist command value I by means of the steering assist command value computing unit 32. Moreover, the steering assist command value I is inputted to a subtracter 30A and is also inputted to a differential compensator 34 of the feed forward system, which is used for increasing a response speed. Deviation (I-i) obtained by the subtracter 30A is inputted to a proportional computation unit 35. Proportional output of the proportional computation unit 35 is inputted to an adder 30B and is also inputted to an integral compensator 36 for improving characteristics of the feedback system. Outputs of the differential compensator 34 and the integral compensator 36 are inputted to the adder 30B, and an addition of the outputs of the proportional computation unit 35 and the compensators 34 and 35 is performed therein. Signal representing a current control value E, which is a result of an addition performed in the adder 30B, is inputted to a motor driving circuit as a motor driving signal. Motor current value i for the motor 20 is detected by a motor current detecting circuit 38 and is further inputted and feedbacked to the subtracter 30A.
Example of the configuration of the motor driving circuit will be described hereinbelow by illustrating thereof in FIG.3. This motor driving circuit 37 consists of: FET gate driving circuit 371 for driving each of field effect transistors (FETs) FET1 to FET4; an H bridge circuit composed of the field effect transistors FET1 to FET4; and a voltage boosting (or booster) power supply 372 for driving high sides of the field effect transistors FET1 and FET2. Field effect transistors FET1 and FET2 are turned ON or OFF in response to PWM signal having a duty ratio D1. Further, in a region in which the duty ratio D1 is small, the field effect transistors FET3 and FET4 are driven in response to PWM signal having a duty ratio D2 defined by a predetermined linear function (D2=a.multidot.D1+b, where a and b are constants). In contrast, in a region in which the duty ratio D1 is large, the field effect transistors FET3 and FET4 are turned ON or OFF in accordance with the direction of rotation of the motor, which is determined according to the sign of PWM signal. For instance, when the field effect transistor FET3 is in a conductive state, electric current flows through the field effect transistor FET1, the motor 20, the transistor FET3 and a resistor R1. Thus, electric current flows through the motor 20 in a positive direction. Further, when the field effect transistor FET4 is in a conductive state, electric current flows through the field effect transistor FET2, the motor 20, the transistor FET4 and a resistor R2. Thus, electric current flows through the motor 20 in a negative direction. Therefore, a signal representing the current control value E is outputted from the adder 30B as PWM output signal. Moreover, the motor current detecting circuit 38 detects the magnitude of a positive direction current on the basis of a drop in voltage developed across the resistor R1. Furthermore, the motor current detecting circuit 38 detects the magnitude of a negative direction current on the basis of a drop in voltage developed across the resistor R2. Motor current value i detected by the motor current detecting circuit 38 is inputted and feedbacked to the subtracter 30A. One of important specifications for the steering performance obtained under the control of the aforementioned control unit concerns the responsiveness of a (power) steering assist unit. Namely, in the case of assuming the emergency steering operation, it is very important how quick a driver or operator can achieve the steering without any sense of incongruity. In the case of the power steering system, the performance of the motor is a dominant factor for the responsiveness. The power supply voltage for the motor is usually limited to a range of voltages (14.5 to 12.0 Volt) supplied from an alternator or a battery. In addition, a restriction is imposed on the size of the motor in conformity with mechanical specifications. Consequently, from the viewpoint of the design of the motor, there is caused a limit to the responsiveness which can be realized by the power steering system.
On the other hand, if the responsiveness becomes a problem, an assist (current) becomes necessary. In the case that the power steering system employs an ordinary power supply which uses both of the alternator and the battery, in accordance with the relation between the ability of the alternator and the current to be supplied to a load, when the current increases, the power supply voltage shifts from a voltage generated by the alternator to a voltage generated by the battery and thus lowers. Consequently, the responsiveness of the motor shifts from a line or segment A to another line B as illustrated in FIG. 4. Therefore, the conventional power steering system has encountered the problem that the necessary responsiveness cannot be obtained.
Further, when performing a quick steering operation so as to avoid danger, steering torque is increased. Thus, it is necessary to generate the assist torque. When driving the motor in such a manner as to surmount the limitations to the ability of the motor, a rapid change in motor current value occurs and consequently, a pulse-like change in the steering torque is caused, as illustrated in FIGS. 5A to 5C. Namely, in the case that a quick steering operation is performed at a time point t1, a motor angular speed or velocity increases as shown in FIG. 5A. In contrast, a motor current i decreases abrupt1y owing to a counter (or back) electromotive force, as illustrated in FIG. 5B. In addition to this, as shown in FIG. 5C, the steering torque T increases rapidly after the time point t1. Further, the natural oscillation of the system is excited. Thus, a phenomenon indicated by reference character A causes a driver to feel a sense of incongruity.